Chronic Venous Insufficiency: This condition is the most common cause of leg swelling, especially in older adults or those with a history of deep vein thrombosis. It occurs when the veins have problems returning blood from the legs to the heart due to issues like faulty valves.

Heart Failure: Leg swelling can be a symptom of heart failure, where the heart is not pumping efficiently, leading to fluid buildup in the legs.
Kidney Disease: Conditions like nephrotic syndrome can cause leg swelling due to the loss of protein in the urine, leading to low albumin levels and fluid retention.
Liver Disease: Cirrhosis and other liver conditions can lead to leg swelling due to hypoalbuminemia and portal hypertension.
Lymphedema: This is a condition characterized by the abnormal collection of protein-rich fluid in the interstitial tissue, leading to swelling of one or more extremities.

Deep Vein Thrombosis (DVT): Although not the most common cause, DVT is a potentially life-threatening condition that requires immediate attention. It can lead to pulmonary embolism if the clot breaks loose.
Cellulitis: A bacterial infection of the skin and subcutaneous tissues that can cause swelling, redness, and warmth of the affected leg.
Compartment Syndrome: A condition where increased pressure within a muscle compartment leads to inadequate blood supply to the tissues, which is a medical emergency.

Sarcoidosis: A condition that can cause leg swelling due to granulomatous inflammation affecting the lymph nodes and other tissues.
Pulmonary Arterial Hypertension: A rare condition characterized by high blood pressure in the arteries of the lungs, which can lead to right heart failure and subsequent leg swelling.
Fabry Disease: A genetic disorder that can cause leg swelling due to the accumulation of a particular type of fat in the body's cells.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and pain.